Bone is a unique tissue that undergoes continuous remodeling throughout life and retains the potential for regeneration even in adult. Bone regeneration is required for bone defects caused by fracture and osteoporosis. Bone morphogenetic proteins (BMPs) are multifunctional growth factors that belong to the transforming growth factor (TGF) superfamily. About 30 BMP-related proteins have been identified and can be subdivided into several groups based on their structures and functions. Especially, BMP2, BMP4 and BMP7 could induce chondrocyte-derived osteoprogenitor (CDOP) cell differentiation, and are important in bone formation and regeneration.
BMPs are synthesized as pre-pro peptides consisting of a signal peptide (SP), latency associated peptide (LAP) and mature peptide (MP). After the synthesis, SP and LAP are processed by enzymatic cleavage, where the C-terminal mature domain is released and secreted. BMPs bind to two-types of BMP receptors and signals through Smad-dependent (canonical) and Smad-independent (non-canonical) pathways. In the canonical pathway, BMP type I receptors phosphorylate receptor-regulated Smads (R-Smads). Phosphorylated R-Smads form a complex compound with common-partner Smads (Co-Smads), translocate into the nucleus and regulate the transcription of osteogenic-related genes.
There are four phases in the process of bone fracture repair: i) inflammatory response, ii) endochondral formation (soft callus formation and osteoblast recruitment), iii) primary bone formation (hard callus formation and mineralization), and iv) secondary bone formation (remodeling). The bone healing process involves various associated factors including BMPs and TGF-β. The effect of BMPs in recombinant systems demonstrates their abilities to enhance fracture healing and skeletal defect repairs in a variety of animal models. Osteogenic potential of BMPs has allowed for their successful use as therapeutic agents for fracture healing, where enhancing bone regeneration has become general practice in spine fusion surgeries and fracture repair. The responsible genes and associated transcription factors for osteogenesis are also activated to express within a few hours of BMP treatment.
The FDA has approved the use of recombinant human BMPs (rhBMPs) including BMP2. However, rhBMPs have rapid systemic clearance and short biological half-life (7 to 16 minutes systemically and up to 8 days locally) and possible negative side-effects (ex. cancer risk) due to high dosage of BMP.